What is the Difference Between Chain Isomerism and Position Isomerism

Isomers are molecules with the same formula but different structures. Various compounds exhibit different types of isomerism, such as positional isomerism and chain isomerism. This small change in the structure can affect the properties of molecules having the same chemical formula.

What is the difference between chain isomerism and position isomerism? Chain isomerism focuses on the carbon skeleton, while position isomerism focuses on functional group placement.

Key Areas Covered

1. What is Chain Isomerism 
      – Definition, Features 
2. What is Position Isomerism
      – Definition, Features 
3. Similarities Between Chain Isomerism and Position Isomerism
      – Outline of Common Features
4. Difference Between Chain Isomerism and Position Isomerism
      – Comparison of Key Differences
5. FAQ: Chain Isomerism and Position Isomerism
      – Answers to Frequently Asked Questions

Key Terms

Chain Isomerism, Position Isomerism

What is Chain Isomerism

Chain isomers have the same molecular formula but different carbon skeletons. Chain isomerism is a type of structural isomerism. The carbon chain in a molecule is considered the backbone of the molecule. In straight-chain isomers, the carbon chain is a simple and branched line. But in chain isomerism, there is much more complexity. Due to the branching of the carbon chain, the molecule gets a more complex structure. This simple difference accounts for major changes in the properties of the isomers.

For example, consider butane (C₄H₁₀). It has different structural isomers having the same molecular formula. It has two isomers: n-butane (straight chain) and isobutane (branched chain).  Due to the way the molecules pack and interact in the two isomers, the n-butane boils at a slightly higher temperature than isobutane.

The number of chain isomers increases with the increase in the number of carbon atoms in the molecule. For example, pentane has three isomers, while hexane has five isomers.

There are many uses of chain isomerism, mainly in the field of organic chemistry. They are found in fuel production, material design, understanding biological processes and in organic synthesis.

What is Position Isomerism

Positional isomer molecules have the same chemical formula and the same functional groups but different positions of the functional groups. Minor changes in the position of the functional group in the carbon chain can cause a major change in the properties of the molecules.

Functional groups, like the hydroxyl (-OH) group or a double bond (C=C), can occupy various positions along the carbon chain, which can be considered the backbone of the molecule. For example, consider butanol (C₄H₁₀O). It has two positional isomers: butan-1-ol, where the hydroxyl group sits at the end of the chain, and butan-2-ol, where it’s attached to the second carbon atom. Due to this difference in the arrangement of the functional groups, properties like boiling point vary significantly between the two types of molecules. For example, butan-1-ol boils at a higher temperature than butan-2-ol. The potential of forming the positional isomers increases as the number of carbon atoms in the carbon chain and the number of functional groups increase.

There are many uses of positional isomerism. They are useful in applications in material science, in making flavours and fragrances and in drug design.

Similarities Between Chain Isomerism and Position Isomerism

1. Both types of isomers share the same number and type of atoms, represented by the same molecular formula.
2. They both differ in arrangement (of the isomers).

Difference Between Chain Isomerism and Position Isomerism

Definition

• Chain isomerism refers to a type of structural isomerism where compounds have the same molecular formula but differ in the arrangement of carbon atoms in the main chain. However, position isomerism refers to a type of structural isomerism where compounds have the same molecular formula but differ in the arrangement or location of functional groups or substituents within the molecule.

Focus

• Chain isomerism focuses on the carbon skeleton, while position isomerism focuses on functional group placement.

Functional Groups

• Chain isomers don’t involve the position of functional groups, while position isomers differ in the position of functional groups attached to the carbon skeleton.

Three Carbon Atoms

• Chain isomers can occur with as few as three carbon atoms, while position isomers typically require at least three carbon atoms and a functional group.

Number of Isomers

• In chain isomers, the number of isomers increases rapidly with the number of carbon atoms. However, in position isomers, the number of isomers depends on the functional group and the number of available positions on the chain.

FAQ: Chain Isomerism and Position Isomerism

1. What is an example of a chain isomerism?

Butane has two chain isomers. One isomer has a straight chain with four carbon atoms in a row (n-butane). The other isomer has a three-carbon chain with a one-carbon branch on the second carbon atom.

2. Do positional isomers have different properties?

Yes, positional isomers can have different properties, especially physical properties.  The physical properties may include boiling point, density, melting point, and viscosity.

3. What are the positional isomers of pentane?

Pentane (C5H12) doesn’t actually have positional isomers. However, pentane does have three structural isomers, which differ in the way the carbon atoms are connected. These isomers are n-pentane, isopentane, and neopentane.

4. What are the chain isomers of Pentanol?

Pentanol has four chain isomers.  They are, pentan-1-ol (n-pentanol),3-Methyl-1-butanol, 2-Methyl-1-butanol and 2,2-Dimethyl-1-propanol.

Reference:

1. “Positional Isomers.” Libre Texts – Chemistry.

Image Courtesy:

1. “Positional isomers of methoxphenidine” By Glucuronide – Created with MarvinSketch (CC BY-SA 3.0) via Commons Wikimedia
2. “Alkane 4 structure” By 1840460mahesh – Own work (CC BY-SA 4.0) via Commons Wikimedia